And the crop factor on four thirds cameras like the E510 is 2x, so to match the 50mm field of view, you'd need a 25mm focal length.

But then there's also a magnification factor in the viewfinder to take into account when you're trying to match what you're seeing... it is a fun experiment anyway to have both eyes open and to adjust the zoom until the view through the viewfinder matches the magnification of the eye...